Nanomaterials inferior to 100 nm are constituent blocks of a new and recently emerging subject area, designated nanotechnology. In fact, an enormous interest all over the world is now observed towards the “very small”. From scientific areas to politics itself, nanotechnology is also frequently present in main world leaders' speeches. It is presented as holding the potential of providing a technological discontinuity with as relevant positive implications in citizen life as past historical events, such as the discovery of steam machines, the train, the automobile, the computer and biotechnology. Such enthusiasm is a result of particles or structures, such as films, pores or nanometric surfaces within a given material, presenting a group of electrical, optical, magnetic and mechanical properties, which are different from those of the same material having superior dimensions. In particular, we refer to properties such as high plasticity, super hardness, lower melting point, transparency and high surface area per mass unit with a consequent improvement in catalytic activity, low thermal conductivity, an increase on the magnetic effect, high semiconductor luminescence, color alterations, and inclusively quantum mechanics laws compliance. Some scientists inclusively propose a theory on the probable state of matter.
In recent years, and as a result of the knowledge on these new properties, a thorough investigation has been observed towards the development of new nanomaterial production methods which comply with two fundamental requirements: i) production yield at an industrial scale (ton/day); ii) reproducibility of nanomaterial properties, essential conditions to support the large scale incorporation of the nanomaterial thus produced, either creating new products or improving the functionalities of those already existing in multiple cross applications such as energy, environment, building constructing, automobile, chemistry, optics, electronics and medicine technical fields.
The known nanomaterial synthesis methods, and which are thoroughly referred to in the literature, are framed within three major categories:
I—Liquid-Phase Methods
This category comprises a group of methods already established or yet in industrialization imminence, namely: a) sol-gel, b) co-precipitation and c) hydrothermal and electrochemical synthesis. These have as common principle a starting material in solution or gel form, wherein precursors are either dissolved to a molecular scale or dispersed in the desired stoichiometric proportion. In a following stage, these precursors are decomposed in controlled manner, a precipitate being formed, usually as an hydroxide, which requires several subsequent treatment stages, such as calcination, for its conversion into an oxide with the desired crystalline structure, being followed by a final breakdown by grinding process.
The major drawback/limitation in these liquid-phase methods are the low production rates (g/h), as well as the high liquid/solvent amounts, which must be later removed or eliminated so as to maintain the nanoparticles in low aggregation. In this removal process, the solvent must be carefully and meticulously separated from the nanoparticles, thus implying compulsory recycling and treatment systems for liquid effluents, which consume plenty of energy and require complex equipments of time consuming operation.
II—Solid-Phase Methods
In this category, nanoparticles are usually prepared from a first slow reaction in the solid state among different precursors, such as carbonates, oxides etc. It is also designated “mechanosynthesis”, wherein the reaction activation energy is supplied by a mill, being followed by an intensive grinding process until particles inferior to 200 nm are obtained. The main limitations in this low-cost method, besides the difficulties observed in obtaining dimensions inferior to 0.2 microns, are related to the presence of impurities, with a non-homogeneous particle-size distribution and essentially at a deficient homogeneity degree, specially when it comes to synthesizing composites and ternary structures or superior structures, originated by incomplete diffusion reactions among reagents, this last limitation having strong consequences in synthesized nanomaterial reproducibility.
III—Gaseous-Phase Methods
These methods comprise processes for both the production of individual nanoparticles and for direct application in surface coating, namely a) combustion synthesis, b) spray pyrolysis, c) evaporation/oxidation of metals, plasma, CVD, PVD, laser deposition, etc.
Generally, there are three production stages:
a) Precursor conversion into vapor, with the formation of an aerosol;
b) Condensation as nanoparticles (with heat release), subsequently to the precursor's oxidation reaction;
c) Control and preservation of nanomaterial dispersion state.
Once it is a “bottom-to-top” approach, the higher the oversaturation state becomes, the smaller the first thermodynamically-stable particles in condensed form (stage b) will be. This oversaturation state is favored by very high pressures or low temperatures (in case of fog formation). On the other hand, in order to avoid undesirable coagulation/coalescence phenomena, which lead to an extremely prompt growth of the particles (stage c), it is necessary to produce extremely dispersed aerosols, which translates into extraordinarily reduced production rates (g/h). The other two alternatives, such as immediate cooling, after nanoparticle condensation or the use of high-speed gas flows and turbulence, have so far demonstrated to be of difficult industrial implementation.
The emulsion detonation is a singular method in nanomaterial synthesis, usually in gaseous phase, containing some highly interesting characteristics which allow overcoming some of the limitations inherent to gaseous-phase synthesis:
a) Extremely high pressures, that might go up to 10 GPa (100,000 bar), turning the first structure of stable condensed matter into very small dimension;
b) Extremely fast cooling, due to the speed of adiabatic expansion of the gases resulting from the reaction;
c) Gas flow with high-speed expansion and turbulence.
These reasons have led to an interest towards the use of the emulsion detonation concept as nanomaterial synthesis method, usually in gaseous phase, a set of recent documents being referenced, which however disclose some weak points as far as class-1 matter use, such as explosives and/or detonators are concerned, which represent a high risk in discontinuous production operations:
EP1577265, “Production of fine powder of aluminium oxide” discloses an industrial process for the production of micrometrical alumina from a cyclic detonation method of mixed granulated aluminum with an oxidizer, the later possibly being an emulsion (w/o). It is a method limited to the obtaining of alumina of micrometrical dimension, and depicts mainly the use of metals as precursors. Previous preparation of class-1 material (explosive) is used, and consequently, a discontinuous process, as well as wet collection associated to extremely complex effluent treatment.
WO2009040770 “Nanocrystalline spherical ceramic oxides, process for the synthesis and uses thereof” discloses a synthesis process for spherical micrometrical particles, with nanocrystalline structure, from the detonation of an emulsions (w/o) at a detonation temperature superior to the melting point of the oxides thus formed, allowing these to assume a spherical form. It is a discontinuous synthesis method in gaseous phase that requires the previous preparation of class-1 emulsions (explosives).
WO2009144665 “Nanometric-sized ceramic materials, process for their synthesis and uses thereof” discloses a method for nanomaterial synthesis, such as binary, ternary and higher oxides, nitrates and carbonates, from an emulsion (w/o) detonation at low temperatures (inferior to 2000° C.), with dissolution of soluble metallic precursors in oxidizing phase (internal), from the addition of soluble propellants to the external phase or addition of metals or metal alloys, after emulsion formation. It is also a discontinuous synthesis method in gaseous phase, also requiring the previous preparation of class-1 emulsions (explosives).
Xiao Hong Wang et al. (Nano-MnFe2O4 powder synthesis by detonation of emulsion explosive. Applied Physics A: Materials Science & Processing. Vol. 90, no. 3, March 2008) discloses nanoparticle synthesis of a ferrite (MnFe2O4) from the detonation of an emulsion (w/o) ignited by a military explosive (RDX), wherein the precursors (iron nitrates and manganese, respectively) were previously dissolved in the internal phase. It is a discontinuous synthesis method in gaseous phase that requires the use of explosive material (class 1), such as RDX and detonators. Xinghua Xie et al. (Detonation synthesis of zinc oxide nanometer powders. Materials Letters, Vol. 60, issues 25-26, November 2006. Pp 3149-3152) discloses a process for obtaining ZnO and Li2O nanoparticles from the detonation of an emulsion, in which lithium and zinc nitrates were dissolved in the internal phase. This emulsion is later ignited by a no. 8 detonator, placed inside a military explosive (RDX). It is a discontinuous synthesis method in gaseous phase, which also resorts to the use of explosive material (class 1), as RDX and detonators, as per in the previous documents.
The process of the present invention is suitable in the category of emulsion detonation and aims at overcoming the limitations above-mentioned and currently inherent to the current methods, namely:
1) Use and/or accumulation of class-1 matter;
2) Procedural discontinuity, usually the composition meant for detonation, after having been prepared and sensitized is manually disposed into a detonation chamber, the ignition being remotely actuated by means of a detonator.
For such purpose, the process of the present invention is aided by a set of technologies, such as:                Sensitization of the emulsion (transformation into class-1 matter) only at the later stage of reactor feeding;        Detonation ignition without detonators or any class-1 matter;        Simultaneous and continuous combination of emulsification and detonation operations of the emulsion.        
As a result, the process of the present invention provides a nanomaterial production yield superior to 100 kg/h, with high reproducibility in an automatic process and with increased safety, once it avoids the use or accumulation of any explosive substances along the whole synthesis process. Nanomaterial collection is carried out in secco, thus avoiding all problems associated to liquid effluent toxicity.